Adsorption of Al3+ to phosphatidylcholine vesicles.

Aluminum toxicity to soil and aquatic organisms is widespread, but the mechanisms of toxicity are unknown. To understand these mechanisms, it is important to know how aluminum reacts with cell surfaces. In this report, we studied adsorption of Al3+ to liposomes composed of phosphatidylcholine, the most abundant phospholipid in plasma membranes of eukaryotic cells. Our equilibrium dialysis and electrophoresis experiments both showed that Al3+ has a 560-fold higher affinity for the phosphatidylcholine surface than Ca2+. Unlike previous reports for adsorption of divalent metals, adsorption of Al3+ to phosphatidylcholine was predicted only approximately by the Stern model. Adsorption of AlF2+ and AlF2+ to the surface was not detectable at the activities we used. From our data, we calculate that Al3+ at 5 x 10(-6) activity could neutralize the surface charge on plant cell plasma membranes and cause a surface potential shift from -30 to +11 mV. This is consistent with non-specific Al3+ inhibition of cation uptake by root cells. Al3+ adsorption to phosphatidylcholine may also play a role in aluminum uptake into cytoplasm by endocytosis.